The present invention relates to a quantization process in an encoding by transformation, particularly orthogonal, for the transmission of image signals. This process constitutes one of the stages of the real time treatment of a sequential information signal, such as a video signal, with a view to compressing the volume of data necessary for representing said signal in order to transmit it with a minimum bit rate, which can in particular be adapted to a 64 kbit/s transmission channel.
The process of the invention more particularly applies to the quantization of television, visioconference and picturephone images or pictures. It can also be used for slow variation images, such as in remote monitoring.
Numerous image encoding systems are known for the purpose of compressing an image signal in order to be able to transmit said image on a transmission line with a reduced bit rate. Reference can e.g. be made to EP-A2-84270, EP-A2-123456 and U.S. Pat. No. 4,196,148, which describe various image encoding systems.
In each encoding system, the image signal received is a digital bidimensional signal. This signal is encoded in four stages:
a transformation stage which, on the basis of the bidimensional signal describing an image in the spatial range, produces a bidimensional signal describing the image in a transformed range (also incorrectly called frequency range), PA0 a stage of quantifying the transformed bidimensional signal to reduce the number of levels of transformed coefficients of the transformed bidimensional signal, PA0 a stage of scanning or sweeping the transformed image to produce a monodimensional sequence of transformed coefficients, PA0 and optionally an encoding stage for encoding the sequence of transformed coefficients, the encoding process making use of a statistical code, e.g., of the Huffman code type.
In practice, the four encoding stages described hereinbefore do not directly apply to the overall image, because the transformation stage would be very complex and would require an exorbitant calculation time. Instead of processing or treating the image all at once, the image has to be broken down into several blocks, to each of which are successively applied the four aforementioned stages.
After transformation, these blocks are coded either in the intra-image, or the inter-image mode. In the first case, the untransformed block represents part of an image, whereas in the second the untransformed block represents a difference between two image portions, which are generally respective an image portion of an image raster and the same image portion of the preceding image raster. Only said image portion is encoded and transmitted.
In the case of inter-image mode encoding, it is optionally possible to have a prediction from the preceding image memory, which is coded, reconstructed and which undergoes filtering.
Each block has a size N.times.N of approximately 8.times.8 pixels, said size generally constituting a good compromise between the complexity of the transformation stage, which increases with the size of the blocks and the rate on the transmission line. In general terms, a block can be rectangular (size N.times.M pixels) and an image can be broken down into blocks not all having the same size.
The transformation operation applied to each block can be a discrete cosine transformation, a Fourier transformation, a Hadamard transformation, a Haar transformation, a highly correlated transformation, etc. The coefficients of the transformed block are generally called frequency coefficients or sequential coefficients for the Hadamard transformation, the coefficients of the transformation block not exactly representing the frequency components of the image corresponding to the untransformed block. However, the term frequency range is incorrectly used for the transformed block range.
In a transformed block of size N.times.M, in which the coefficients have row indexes between 0 and N-1 and column indexes between 0 N-1, the coefficient of coordinates (0,0) represents the continuous component and the other coefficients the alternating components, the low frequency components corresponding to the low index coefficients and the high frequency components to the high index coefficients.
It is known that the continuous component generally has a high value and on average the value of the alternating components decreases on passing from low to high frequencies.
The quantization of the values of the transformed coefficients brings about a first compression of the block. For this quantification, quantization levels are defined (also called decision levels), which are regularly or irregularly distributed in the range [-B,B], in which B is the maximum possible amplitude for a transformed coefficient.
Thus, quantization consists of defining a measuring scale constituted by several levels spaced by the quantization step G, the first being called the quantization threshold. Each coefficient is compared with this quantization threshold.
In more general terms, a comparison takes place between each scanned coefficient and the interval defined by ]+G,-G[.
The coefficients in the interval are coded by a zero.
The coefficients outside the interval are compared with multiples of +G and -G, said multiples defining the different levels, the coefficients then being encoded as a function of these levels. For example, when a coefficient is between 3G and 4G, its encoding will correspond to an amplitude 4.
As certain blocks contain more or less informations linked with movements, there is a regulation between the information quantity to be transmitted of one block of an image with respect to a block of the following image. For this purpose there is a suppression of the low frequency coefficients in the blocks corresponding to many movements in the image. With the same aim, it is also possible to modify the quantization step G from one block of an image to a block of the following image and e.g. choose a low step G for blocks without movement and a high step G for blocks with significant movements.
All the aforementioned known treatments are described in the aforementioned patent applications and also in the article entitled "L'image numerique et le codage", 4th quarter 1986, No. 126 Echo des Recherches and relate to significant compression of an image signal, whilst maintaining an appropriate image quality.
However, with the aim of compressing the information quantity to be transmitted in order to reduce the flow rate and carry out a low rate transmission, as performed in the digital telephone network at 64 kbit/s, the Applicant has not merely observed the presence or absence of movement from one image to the next and therefore from one block to the next, as has been done in the prior art.
The Applicant has unexpectedly had the idea of acting on the quantization step G within the actual coefficient block, in order to modify the threshold between the different coefficients of the same block. The Applicant found on the basis of tests and a practical realization that the variation of the quantization step within the block makes it possible to further compress the information to be transmitted without leading to any deterioration in the transmitted image quality.
The present invention therefore relates to a threshold quantization process in encoding by transformation for the transmission of image signals consisting of varying the quantization threshold within the scanned coefficient block.